log: enhance setting loglevels

My apologies for the patch, that was totally wrong. Here is a proper patch:

https://github.com/Darkvater/minidlna/compare/master...%23111-log_level.diff

I've written tests for it as well, don't really know where to put them. It allows a lot more flexibility:
* 'log_level=debug' => set the level for all categories in one go
* 'log_level=ssdp=debug' => set level for single category leaving everything else at default
* 'log_level=debug,ssdp=error' => set default, override for single category
* and all the original options still work